It looks like an US M4 carbine, but the Canadian-made Diemaco C8 carbine looks practically identical and that particular rifle has the Canadian-issue Elcan scope on it. I have no idea whose version of the M203 that is.

edit: I'd bet that it's a US M4. Why it has the Canuk scope on it, I have no idea.

They do. I believe that he is refering to the fact that the dome is oriented so that if snow falls on it, the weight of the internals isn't off-balanced, causing more stress than necessary. But that's just a guess.